Kosoado stands Kore, Sore, Are, and Dore which is part of the Kotoba 言葉 or sentence. In English class, we know the kind of words shows this, that, and Which, while in Japanese, there are 4 types of the point, ie kore, sore, are and dore, which is known as kosoado kotoba (こそ あど 言葉)